Determination of Physicochemical Properties of Ore-Forming Fluids Using Combined Mineralogy and Fluid Inclusion Microthermometric Studies in the Ravanj Pb-Ba-Ag Deposit, Delijan, Markazi Province

Located 20 km north of the Delijan city، the Pb-Ba-Ag ore deposit was mineralized in the lower Cretaceous carbonate host rocks in the Ravanj anticline. Geographically، the Ravanj anticline is part of the Urumieh-Dokhtar magmatic arc in the Zagros orogenic belt. Deposition of the ore took place in the lower part of massive limestones where they have been structurally thrusted over the shale and shale-limestone strata. Breccia filling، host rock replacement and disseminated ore are the main textural features of mineralization. Mineralization consists of fine-grained galena، barite، variable amounts of pyrite، and minor amounts of sphalerite، tetrahedrite، and chalcopyrite. Despite extensive pyritization، marcasite was not found in the ore، indicating that the ores were mineralized from a fluid having a pH > 5. Fluid inclusion microthermometric studies were done in the calcites of pre-main-stage mineralization (C2)، in main-stage barite and in post-mineralization calcite (C3). Average homogenization temperatures of fluid inclusions are approximately equal: 165 ˚C in the pre- main stage calcite، 160 ˚C in post-mineralization calcite and 175 ˚C in barite، but their salinities change from lower than 1 to higher than 18wt% NaCl equivalent. Silica precipitation in the Ravanj deposit is very limited، in agreement with minor changes in temperature of fluid during mineralization. The wide range in salinity of the fluid inclusions plus contemporaneous deposition of barite and fine-grained galena are evidences for mixing of two geochemically different fluids. One of them was probably a low-salinity (5. 6 wt% NaCl)، CO2 -beaing and sulfur-rich fluid. At a temperature of 160 ˚C، the neutral pH is about 5. 8. Therefore the sulfides were deposited from fluids having a pH of 5-6. The effect of low-salinity، CO2 -beaing fluid is to buffer the system. The second fluid، which was probably oxidized، saline (15. 7 wt% NaCl) and metal-rich، shows salinity and homogenization temperatures characteristic of MVT ore forming fluids.